Recent mobile communication systems include various base stations such as macrocell base stations, picocell base stations, and femtocell base stations. A macrocell base station forms a macrocell having a cell radius of 1 km or more, a picocell base station forms a picocell having a cell radius on the order of 0.5 km-1.5 km, and a femtocell base station forms a femtocell having a cell radius on the order of 10 m-500 m. Of these, a femtocell and a picocell in particular are referred to as small cells. In the present specification, the term “small cell” is used as appropriate.
There are generally two purposes for forming small cells: the first purpose being to supplement coverage, and the second purpose being to increase capacity.
Regarding the first purpose, the radio waves of a macrocell base station are lost due to walls of buildings and have difficulty reaching the interior of a residence. A base station that forms a small cell is therefore installed inside a residence, and this base station, by emitting radio waves, enables the reception of adequate mobile communication service even inside the residence.
Regarding the second purpose, in recent years, user traffic (user data, of which packet data are representative) has soared with the proliferation of portable telephones and the rise of smartphones. In small cells, reducing the cell radius of a base station enables a decrease of the number of people that are accommodated per cell and increases the overall capacity. In this way, the small-cell solution of reducing the cell radius of a base station is thus becoming more widespread because it can deal with the rapidly rising user traffic.
A mobile communication system in which small cells are applied is next described.
FIG. 1 shows a summary of the configuration of an LTE (Long Term Evolution) mobile communication system. FIG. 1 shows a system that allows activation of SIPTO (Selected IP Traffic Offload, where IP is an abbreviation of (Internet Protocol) above RAN (Radio Access Network), to be described hereinbelow.
In FIG. 1, UE (User Equipment) 101 and 102 are portable terminals.
S-GW (Serving-Gateways) 108, 111, and 117 are devices that transmit user data (U (User)-plane) in a core network.
P-GW (PDN-Gateway, PDN: Packet Data Network) 107, 112, and 119 are devices having an interface with an EPC (Evolved Packet Core), an IMS (IP Multimedia Subsystem), or an outside packet network (for example, outside networks such as Internet 106, 113, and 120).
MME (Mobility Management Entity) 115 is a core network device that performs signal control and mobility management of UEs 101 and 102 in a core network and selects the paths of user data (i.e., S-GW and P-GW) with UEs 101 and 102.
eNB (evolved NodeB) is a base station that performs wireless communication with UE 101.
HeNB (Home eNB) 104 is a base station that carries out wireless communication with UE 102.
HeNB 104 indicates an LTE femtocell base station, and eNB 103 indicates an LTE base station other than a femtocell base station, and may be a picocell base station or a macrocell base station.
Although not shown in FIG. 1, HNB (Home NodeB) indicates a 3G (3rd Generation) femtocell base station, NodeB indicates a 3G base station other than a femtocell base station, and may be a picocell base station or a macrocell base station.
Although not shown in FIG. 1, the following abbreviations are used as appropriate in the present specification.
(H)eNB is assumed to indicate either HeNB or eNB.
H(e)NB is assumed to indicate either HNB or HeNB.
(H)(e)NB is assumed to indicate any of HNB, HeNB, NodeB, and eNB.
HeNB-GW 114 accommodates a plurality of eNB 103 or a plurality of HeNB 104, is a gateway device that connects these devices to a core network, and relays user data and control signals (C (control)-plane) between a core network and HeNB 104 or eNB 103. In addition, HeNB-GW 114 may also accommodate a base station that is equipped with other wireless communication capacities such as WiFi (Wireless Fidelity).
Although not shown in FIG. 1, a gateway device that accommodates a 3G femtocell base station is referred to as HNB-GW.
SeGW (Security Gateways) 109 and 110 establish IPsec tunnels with HeNB 104 and eNB 103 and provide secure communication.
HSS (Home Subscriber Server) 116 holds information for each subscriber that uses UEs 101 and 102 and when there is an inquiry for information relating to a subscriber from MME 115, returns this information.
DNS (Domain Name System) 118 is used when MME 115 selects S-GW or P-GW.
MME 115 uses the DNS mechanism of the related art on the basis of information of the TAC (Tracking Area Code) and RAC (Routing Area Code) of a base station that UEs 101 and 102 are accessing to select S-GW or P-GW that transmits the user data of UEs 101 and 102.
In addition, when unable to select an appropriate S-GW or P-GW by only information of the TAC and RAC, MME 115 takes into consideration information such as the RNC (Radio Network Controller), RNC-ID of the eNB, or eNB-ID that UEs 101 and 102 are accessing and uses the DNS mechanism of the related art to select the S-GW or P-GW that transmits the user data of UEs 101 and 102.
As described hereinabove, the present system is capable of activating SIPTO above RAN. In SIPTO, user data that were transmitted to the Internet from a UE are assumed to be offloaded at an offload point that is close to the base station that was accessed by the UE.
In FIG. 1, three S-GW 108, 111, and 117 are shown as S-GW.
Of these S-GW, S-GW 117 is a node in the core network and is used when SIPTO is not activated.
S-GW 108 is an S-GW for offloading that is selected as the offload point when SIPTO is activated and is the S-GW that is geographically/or network-topologically closest as seen from eNB 103 when SIPTO is activated under the condition in which UE 101 is accessing eNB 103.
S-GW 111 is the S-GW for offloading that is selected as the offload point when SIPTO is activated and is the S-GW that is geographically/network-topologically closest as seen from HeNB 104 when SIPTO is activated under the condition in which UE 102 is accessing HeNB 104.
In FIG. 1, three P-GW 107, 112, and 119 are shown as P-GW.
Of these P-GW, P-GW 119 is a node in the core network and is used when SIPTO is not activated.
P-GW 107 is a P-GW for offloading that is selected as the offload point when SIPTO is activated and is the P-GW that is geographically/network-topologically closest as seen from eNB 103 when SIPTO is activated under the condition in which UE 101 is accessing eNB 103.
P-GW is a P-GW for offloading that is selected as the offload point when SIPTO is activated and is the P-GW that is geographically/network-topologically closest as seen from HeNB 104 when SIPTO is activated under the condition in which UE 102 is accessing HeNB 104.
SeGW 109 and 110 may be installed in the core network, or may be installed in Backhaul Network 105. No particular limitation applies to the installation locations of SeGW 109 and 110.
Similarly, HeNB-GW 114 may be installed in the core network or may be installed in Backhaul Network 105. No particular limitations apply to the installation location of HeNB-GW 114.
Similarly, S-GW 108 and 111 and P-GW 107 and 112 for offloading may be installed in the core network or may be installed in Backhaul Network 105. In addition, the functions of S-GW and P-GW for offloading may be provided in HeNB 104 and eNB 103. No particular limitations apply to the installation locations of S-GW 108 and 111 or P-GW 107 and 112 for offloading.
HeNB-GW 114 is next described in detail.
A gateway device that accommodates a base station that forms small cells is referred to as a small-cell gateway. HeNB-GW 114 is here described as a small-cell gateway that accommodates a plurality of HeNB 104.
Installing HeNB-GW 114 as a small-cell gateway enables not only line concentration of the C-plane of the S1 interface (abbreviated as S1-C) with MME 115 as shown in the related art (3GPP TS36.300 Ver11.5.0, 3GPP: 3rd Generation Partnership Project) but also enables line concentration of the U-plane of the S1 interface (abbreviated as S1-U) with S-GW 117.
Regarding the C-plane, MME 115 carries out transmission and reception of a HEARTBEAT signal of SCTP (Stream Control Transmission Protocol) in order to monitor the state of the link on the S1 interface. However, the bundling of a plurality of HeNB 104 by HeNB-GW 114 eliminates the need for MME 115 to transmit and receive the HEARTBEAT signal with each HeNB 104, and MME 115 need only transmit and receive the HEARTBEAT signal with HeNB-GW 114. In other words, HeNB-GW 114 monitors the state of links by transmitting and receiving the SCTP HEARTBEAT signal with each HeNB 104, whereby the signal load of MME 115 can be reduced.
Regarding the U-plane on the other hand, S-GW 117 monitors the normalcy of the path of GTP-U by an ECHO procedure of the GTP-U (GPRS Tunneling Protocol-User where GPRS is General Packet Radio Service) protocol. However, the bundling of a plurality of HeNB 104 by HeNB-GW 114 eliminates the need for S-GW 117 to implement the ECHO procedure with each HeNB 104, and S-GW 117 need only implement the ECHO procedure with HeNB-GW 114. In other words, HeNB-GW 114 monitors the state of the U-plane by implementing the ECHO procedure with each HeNB 104, and as a result, the signal load upon S-GW 117 can be reduced.
In the core network, moreover, IP addresses have already been assigned to apparatuses in the core network. When a large number (on the order of, for example, several 100,000) of HeNB 104 are subsequently introduced, the possibility arises that in some cases restructuring such as the re-assignment of IP addresses will become necessary due to the limitation of IP addresses that can be assigned by the provider.
However, installation of HeNB-GW 114 between HeNB 104 and the core network enables the user data and control signals that are transmitted by way of HeNB-GW 114 to be first terminated at HeNB-GW 114 and allows the addresses to be replaced by the address of HeNB-GW 114. In this way, only the IP address of HeNB-GW 114 need be prepared in the core network and the independence of the IP address space between the core network and HeNB 104 can be maintained. In this way, the existence of HeNB-GW 114 allows the easy introduction of HeNB 104 without any influence upon the setting of IP addresses of the core network. HeNB-GW 114 therefore realizes a concentration function for a large number of HeNB 104.
In addition, because it is assumed that HeNB 104 will be installed in residences and businesses, it is also assumed that the power will be more frequently turned ON and OFF than in a macrocell base station. If HeNB-GW 114 is not present at such times, many alarms will be raised in MME 115 and S-GW 117 when the power of HeNB 104 is turned OFF due to faults of SCTP links between MME 115 and HeNB 104 and faults of the U-plane between S-GW 117 and HeNB 104. However, if HeNB-GW 114 is present, there will be no effect upon SCTP links with the MME 115 side or upon the U-plane with the S-GW 117 side even when HeNB 104 is turned OFF, and alarms will therefore not occur.
Thus, HeNB-GW 114 becomes necessary in a state in which a large number of HeNB 104 are installed both to reduce the signal load upon MME 115 and S-GW 117 and to eliminate alarms to MME 115 and S-GW 117 when turning OFF power to HeNB 104.